An EEG signal is derived from electrodes placed on the head of a subject and is conveyed via leads to processing circuitry and monitoring equipment. The EEG signals themselves are relatively weak. A similar situation applies to other kinds of electrophysiological measurement (EPM). These techniques are therefore prone to interference arising from the subject, e.g. eye blink artefacts or from the surrounding environment, e.g. electrical mains interference. An EEG signal obtained from a scalp electrode is in the range typically of 10 μV to 100 μV at an impedance of around 500Ω to 50 KΩ.
EEG has traditionally been used for investigations into brain activity. It may, for example, be employed to investigate abnormal brain activity in disease states such as epilepsy or in certain psychiatric abnormalities.
There is also an interest in obtaining EEG measurements in combination with techniques which involve use of a strong magnetic field and sometimes also radiofrequency (rf) fields. One such technique is functional magnetic resonance imaging (fMRI). Such magnetic and rf fields represent yet another source of interference for the EEG measurements. The large magnetic and radio frequency fields produced by MRI machines easily swamp the EEG signal with induced noise on the signal wire. Further, switching of the MRI magnetic gradients causes extraneous pulses in the EEG signal.
The fMRI technique is widely used in both medical and non-medical imaging to obtain a spatial image of “slices” through the brain. In the medical context, MRI is used to identify lesions such as areas of restricted blood flow or tumours. Outside the medical field, fMRI has, for example, been a useful tool in cognitive neuroscience for investigating brain response to various external stimuli. However, there are other techniques which involve use of a strong magnetic field and may also be used in combination with EEG, as will be mentioned further hereinbelow. They also give rise to noise problems.
There have been many proposals for reducing interference signals in EEG. For example, U.S. Pat. No. 5,445,162 proposes a system using electrodes and wiring designed to minimise noise in EEG signals when obtained in combination with use of fMRI. The EEG recording equipment is located outside the MRI room to minimise interference.
U.S. Pat. No. 5,513,649 proposes a system for removing contaminants from EEG recordings. It proposes that an adaptive filter is used to estimate the contaminants in the measured EEG data and then subtracts them from the primary signal to obtain the corrected EEG data.
WO-A-03/073929 discusses the potential problems associated with concurrent fMRI and EEG measurements, namely noise induced in the EEG signal by the rf and magnetic fields (as mentioned above) and the disruption to the fMRI measurement by introduction of ferromagnetic material in the EEG electrodes, into the bore of the fMRI machine. This reference comments upon possibilities for alleviating these problems. One is to dispense with ferromagnetic materials in the EEG electrodes and to use an alternative such as carbon fibre. Another is to rearrange the EEG leads to minimise interference with the rf field.
The aforementioned WO-A-03/073929 also recognises safety problems inherent in deploying EEG equipment inside a pulsed rf field, e.g. due to induced currents. Solutions to these problems have included raising the impedance of the EEG detection circuit by means of resistors or by using different electrode systems or different electrode materials, or by incorporating a fibre optic link in the line between the electrodes and the circuit. The reference proposes that a better method of avoiding such hazards is to incorporate an amplifier within the electrode structure.
WO-A-02/13689 describes a method of reducing interference in EEG, ECG and EMG, especially in combination with MRI, whereby pairs of electrodes are connected to differential amplifiers. An interference signal is obtained by synchronisation of measurement signals with a timing signal which initiates digitisation of the signals. Subtraction of the interference is then effected digitally.
A system wherein separate EEG measurement signal electrodes and so-called reference electrodes are employed is disclosed in International Patent Application No. PCT/EP2005/006126, unpublished at the priority filing date of this application. The reference electrodes are electrically isolated from the subject. Signals from the individual reference electrodes are subtracted from those on respective measurement signal electrodes to remove interference. One preferred means of supporting the measurement signal electrodes and reference electrodes as disclosed in this reference, is in the form of an electrode support or cap. The reference electrodes comprise connections or “nodes”, electrically connected to a continuous conductive web or mesh which is electrically isolated from the subject by an insulating layer. This conductive layer may be formed from carbon-loaded fabrics, foam or yarn or a silver coated polymer such as nylon.
The present invention, in one aspect, is concerned with an alternative design of electrode cap which is optimised for use with the noise cancellation system disclosed in PCT/EP2005/006126 but which embodies several novel and inventive features. In another aspect, it relates to an electrode cap design for improving contact with the subject and minimising movement of the electrodes relative to the head of the subject.
It may be noted that there have been a number of prior proposals for electrode support caps for electrodes used in EEG measurement. EP-A-0 541 393 discloses such a support in the form of a headpiece which comprises elastic strips on which the electrodes are supported.
WO-A-00/27279 discloses a stretchable elastic cap on which are supported, soft rubber electrode holders.
US-A-2002/0007128 discloses a system for combined EEG monitoring with simultaneous transcranial magnetic stimulation (TMS). Disclosed in this reference is an electrode system using a conductive plastic electrode cup with an integral silver epoxy coated electrode to ensure that the impedance of the electrode system as a whole is reduced to be equivalent to that of a typical metal electrode.
An electrode cap in which sensing electrodes are replaced with electrical devices adapted both for measuring electric voltage or current, and also for applying an electric current or voltage is described in U.S. Pat. No. 6,594,521.